1. Field of the Invention
The present invention relates to a signal detecting method and a signal detecting device using an integrating amplifier for detecting charge signals by correlated double sampling.
2. Description of the Related Art
Conventionally, image information retrieving apparatuses, which retrieve image information by use of photoelectric converter elements such as CCD""s or photomultipliers or solid state image detectors, have been utilized in various fields.
Particularly in the field of medical science, there has been disclosed a solid state radiation image detector capable of recording radiation image information on an electric accumulator as an electrostatic latent image by means of accumulating electric charges in an amount corresponding to a dosage of X-rays irradiated with a radiation image capturing apparatus or the like as electric charges for a latent image, and capable of retrieving the radiation image information by scanning with a laser beam or a line light source as retrieving light.
Moreover, in the above-mentioned image information retrieving apparatus, radiation image capturing apparatus and the like, integrating amplifiers are generally used for detecting retrieved image signals because the integrating amplifiers are processible into integrated circuits and generate a relatively small amount of noise. Such an integrating amplifier is designed to initiate accumulation of electric charges when switched to an accumulator mode, to discharge the accumulated electric charges when switched to a reset mode and thereby to output electric signals corresponding to the amount of the electric charges.
Here, immediately after the integrating amplifier is switched to the accumulator mode, an offset called a charge feedthrough attributable to capacitance of a switch inside the integrating amplifier is outputted as shown in FIG. 8. The charge feedthrough is not always constant but is fluctuant; therefore, it is not possible to obtain accurate signal components corresponding to the image information if processing is applied that subtracts an uniform offset. In this context, processing called correlated double sampling is applied in order to eliminate the influence of charge feedthrough. Correlated double sampling refers to processing, which can eliminate the influence of the charge feedthrough by means of measuring a difference between an electric signal to be outputted immediately after the integrating amplifier is switched to the accumulator mode and an electric signal to be outputted immediately before the integrating amplifier is switched to the reset mode, and by defining the difference as a signal component.
Now, there are two methods of correlated double sampling, namely, analog correlated double sampling (ACDS) and digital correlated double sampling (DCDS).
The analog correlated double sampling uses a readout circuit as illustrated in FIG. 9 and is operated under control corresponding to a timing chart as shown in FIG. 10. The readout circuit includes an integrating amplifier 1, first signal retaining means 2 for retaining an electric signal to be outputted immediately after switching to an accumulator mode, second signal retaining means 3 for retaining an electric signal to be outputted immediately before switching to a reset mode, a differentiator circuit 4 for finding a difference between the electric signals retained in the first signal retaining means 2 and in the second signal retaining means 3 and for outputting the difference as a signal component, and an A/D converter 5 for converting the signal component into a digital signal.
As shown in FIG. 10, a first electric signal to be outputted immediately after the integrating amplifier 1 is switched to the accumulator mode is retained by the first signal retaining means 2 first, and after passage of a sufficient time period for accumulating electric charges in the integrating amplifier 1, a second electric signal to be outputted immediately before the integrating amplifier 1 is switched to the reset mode is retained by the second signal retaining means 3. Then, the difference between the first electric signal and the second electric signal is determined as the signal component with the differentiator circuit 4 and the signal component is converted and outputted by the A/D converter 5. Note that the timing chart illustrated in FIG. 10 shows the control timing for detecting signals for each pixel.
Meanwhile, the digital correlated double sampling uses a readout circuit as illustrated in FIG. 11 and is operated under control corresponding to a timing chart as shown in FIG. 12. The readout circuit includes an integrating amplifier 6, signal retaining means 7 for retaining a first electric signal to be outputted immediately after switching to an accumulator mode and a second electric signal to be outputted immediately before switching to a reset mode, and an A/D converter 8 for converting the signals retained by the signal retaining means 7 into digital signals.
As shown in FIG. 12, the first electric signal to be outputted immediately after the integrating amplifier 6 is switched to the accumulator mode is retained by the signal retaining means 7 first, and the retained first electric signal is outputted by the A/D converter 8 as a first digital signal. Then, after passage of a sufficient time period for accumulating electric charges, the second electric signal to be outputted immediately before the integrating amplifier 1 is switched to the reset mode is retained by the signal retaining means 7. Thereafter, the retained second electric signal is converted into a second digital signal and outputted by the A/D converter 8. Note that a difference between the first digital signal and the second digital signal is computed by software loaded on an image processing apparatus or the like to be connected subsequent to the A/D converter 8.
However, the analog correlated double sampling has the following problem. Specifically, when radiation image information is read out of the radiation image detector while accumulating the electric charge one-on-one for each pixel in the integrating amplifier, for example, the first signal retaining means 2 and the second signal retaining means 3 must retain the first electric signal and the second electric signal during digitalization of the signal component by the A/D converter 5. Accordingly, accumulation of an electric charge corresponding to a subsequent pixel cannot be initiated before A/D conversion of a first pixel is completed. Therefore, a long processing time period is required for obtaining a digital signal for one pixel, because the required time period combines a time period for accumulating the electric charge and a time period for A/D conversion.
Similarly, the digital correlated double sampling has the following problem. When the radiation image information is read out of the radiation image detector while accumulating the electric charge one-on-one for each pixel in the integrating amplifier, for example, the signal retaining means 7 must retain the second electric signal until digitalization of the second electric signal is completed. Accordingly, accumulation of an electric charge corresponding to a subsequent pixel cannot be initiated beforehand. Therefore, a long processing time period is required for obtaining a digital signal for one pixel as similar to the above-described case with the analog correlated double sampling, as the required time period combines a time period for accumulating the electric charge and a time period for A/D conversion.
Moreover, in the case of retrieving the radiation image by scanning the retrieving light as previously described, the time period for accumulating the electric charge needs to be prolonged because speed of response to the retrieving light is slow. Nevertheless, such processing time is preferred to be as short as possible.
Furthermore, in the case of retrieving the radiation image by scanning the retrieving light as described above, it is not possible to retrieve charge signals outputted during A/D conversion. Accordingly, the charge signals during such a period are overlooked, and the S/N ratio of the retrieved radiation image is resultantly deteriorated.
In consideration of the foregoing problems, it is an object of the present invention to provide a signal detecting method and a signal detecting device capable of shortening process time upon detecting the charge signals read out of the radiation image detector or the like by correlated double sampling using an integrating amplifier, and capable of avoiding occurrence of deterioration in the S/N ratio of the radiation image as described above.
A first signal detecting method of the present invention is a signal detecting method of repeating the processes of initiating accumulation of charge signals by switching an integrating amplifier to an accumulator mode, retaining a first electric signal outputted from the integrating amplifier immediately after the integrating amplifier is switched to the accumulator mode, finding a difference between a second electric signal outputted from the integrating amplifier immediately before the integrating amplifier is switched to a reset mode after completing accumulation of the charge signals and the first electric signal retained by the integrating amplifier to define the difference as a signal component, and converting the signal component into a digital signal and thereby outputting the digital signal. Here, the signal detecting method includes the steps of retaining the signal component obtained in connection with a first charge signal, converting the retained signal component into the digital signal, and switching the integrating amplifier to the accumulator mode after completing accumulation concerning the first charge signal but before completing conversion into the digital signal so as to initiate accumulation concerning a second charge signal.
Here, the first signal detecting method refers to a signal detecting method concerning the so-called analog correlated double sampling as described above.
Moreover, the foregoing xe2x80x9ccharge signalsxe2x80x9d refer to signals obtained by converting image information with a photoelectric converter element such as a CCD or a photomultiplier, or signals read out of the solid state radiation image detector, for example.
Moreover, the foregoing expression of xe2x80x9cimmediately after the integrating amplifier is switched to the accumulator modexe2x80x9d may be defined as a moment simultaneous with a moment of switching to the accumulator mode or a moment after passage of a certain time period after switching to the accumulator mode. However, it is preferred that such a moment takes place after settlement of a charge feedthrough phenomenon, which occurs in the event of switching from the reset mode to the accumulator mode.
Moreover, the foregoing expression of xe2x80x9cafter completing accumulation of the charge signalsxe2x80x9d refers to passage of a predetermined accumulation time period of the integrating amplifier.
Moreover, the foregoing expression of xe2x80x9cimmediately before the integrating amplifier is switched to a reset modexe2x80x9d is preferably set as a moment in advance of time for switching to the reset mode just by a period required for confirmation of the retention.
Moreover, if the charge signals are the signals photoelectrically converted with the CCD, for example, then the foregoing xe2x80x9cfirst charge signalxe2x80x9d refers to a charge signal outputted from the CCD in response to a first pixel, and the foregoing xe2x80x9csecond charge signalxe2x80x9d refers to a charge signal outputted subsequently from the CCD in response to a second pixel adjacent to the first pixel. Meanwhile, if the charge signals are the signals retrieved by irradiating linear retrieving light with a line light source onto a solid state radiation image detector, then the xe2x80x9cfirst charge signalxe2x80x9d refers to a charge signal retrieved by irradiating the retrieving light onto a first line, and the xe2x80x9csecond charge signalxe2x80x9d refers to a charge signal retrieved subsequently by irradiating the retrieving light onto a second line adjacent to the first line.
A second signal detecting method of the present invention is a signal detecting method of repeating the processes of initiating accumulation of charge signals by switching an integrating amplifier to an accumulator mode, retaining a first electric signal outputted from the integrating amplifier immediately after the integrating amplifier is switched to the accumulator mode by signal retaining means, converting the retained first electric signal into a first digital signal, retaining a second electric signal outputted from the integrating amplifier immediately before the integrating amplifier is switched to a reset mode after completing accumulation of the charge signals, and converting the retained second signal into a second digital signal and thereby outputting the second digital signal. Here, the signal detecting method includes the steps of retaining the first electric signal obtained in connection with a first charge signal by first signal retaining means, retaining the second electric signal obtained in connection with the first charge signal by second signal retaining means, and switching the integrating amplifier to the accumulator mode after completing accumulation concerning the first charge signal but before completing conversion of the second electric signal into the second digital signal so as to initiate accumulation concerning a second charge signal.
Here, the second signal detecting method refers to a signal detecting method concerning the so-called digital correlated double sampling as described above.
Moreover, the foregoing xe2x80x9csignal retaining meansxe2x80x9d refers to a device including a switch and a capacitor, for example, so that the device can retain the electric signals by accumulating electric charges in the capacitor while turning the switch on.
Moreover, the foregoing expression of xe2x80x9cafter completing accumulation concerning the first charge signalxe2x80x9d refers to passage of a predetermined accumulation time period of the integrating amplifier. However, the predetermined accumulation time period of the integrating amplifier is set longer than a time period required for converting the electric signal into the digital signal. In other words, when the accumulation time period concerning the first charge signal has passed, conversion of the first electric signal concerning the first charge signal into the first digital signal is to be completed already.
A third signal detecting method of the present invention is a signal detecting method of repeating the processes of initiating accumulation of charge signals by switching an integrating amplifier to an accumulator mode, retaining a first electric signal outputted from the integrating amplifier immediately after the integrating amplifier is switched to the accumulator mode by signal retaining means, converting the retained first electric signal into a first digital signal by converting means, retaining a second electric signal outputted from the integrating amplifier immediately before the integrating amplifier is switched to a reset mode after completing accumulation of the charge signals by signal retaining means, and converting the retained second electric signal into a second digital signal by the converting means and thereby outputting the second digital signal. Here, the signal detecting method includes the steps of retaining the first electric signal by first signal retaining means, converting the retained first electric signal into a first digital signal by first converting means, retaining the second electric signal by second signal retaining means, converting the retained second electric signal into a second digital signal by second converting means, switching the integrating amplifier to the accumulator mode after completing accumulation concerning the first charge signal but before completing conversion of the retained second electric signal into the second digital signal so as to initiate accumulation concerning a second charge signal, retaining the first electric signal concerning the second charge signal to be outputted from the integrating amplifier immediately after the integrating amplifier is switched to the accumulator mode, and initiating conversion of the first electric signal concerning the second charge signal into the first digital signal before completing conversion of the second electric signal concerning the first charge signal into the second digital signal.
Here, the third signal detecting method refers to the signal detecting method concerning the so-called digital correlated double sampling as similar to the second signal detecting method.
A first signal detecting device of the present invention includes an integrating amplifier for accumulating charge signals and for outputting electric signals corresponding to amounts of the accumulated electric charges, switching means for switching the integrating amplifier between an accumulator mode and a reset mode, first signal retaining means for retaining a first electric signal outputted immediately after the integrating amplifier is switched to the accumulator mode by the switching means, a differentiator circuit for finding a difference between a second electric signal outputted from the integrating amplifier immediately before the integrating amplifier is switched to the reset mode by the switching means and the first electric signal retained by the first signal retaining means and thereby outputting the difference as a signal component, second signal retaining means for retaining the signal component outputted from the differentiator circuit, and converting means for converting the signal component retained by the second signal retaining means into a digital signal.
Here, the first signal detecting device relates to a device for performing the so-called analog correlated double sampling. Accordingly, it is preferred that the first signal detecting device conducts signal detection in accordance with the first signal detecting method.
Moreover, in the first signal detecting device, the integrating amplifier, the first signal retaining means, the differentiator circuit and the second signal retaining means may be respectively provided on each of line electrodes of an image detector. Here, the image detector is formed by serially stacking a first electrode layer having transmissivity with respect to an electromagnetic wave for recording, a recording photoconductive layer which takes on conductivity by receiving irradiation of the electromagnetic wave for recording transmitted through the first electrode layer, an electric accumulator for accumulating electric charges generated in the recording photoconductive layer by irradiation of the electromagnetic wave for recording as electric charges for a latent image, a retrieving photoconductive layer which takes on conductivity by receiving irradiation of an electromagnetic wave for retrieval, and a second electrode layer having a stripe electrode composed of a plurality of line electrodes being arranged in given pitches for retrieving the electric charges for a latent image from the electric accumulator. Here, the first signal detecting device may also include a multiplexer for switching the signal components concerning the charge signals outputted from the respective line electrodes depending on the respective line electrodes and thereby outputting the signal components to the converting means.
Here, the foregoing xe2x80x9cgiven pitchesxe2x80x9d refer to pixel pitches.
A second signal detecting device of the present invention includes an integrating amplifier for accumulating charge signals and for outputting electric signals corresponding to amounts of the accumulated electric charges, switching means for switching the integrating amplifier between an accumulator mode and a reset mode, first signal retaining means for retaining a first electric signal outputted immediately after the integrating amplifier is switched to the accumulator mode, second signal retaining means for retaining a second electric signal outputted from the integrating amplifier immediately before the integrating amplifier is switched to the reset mode by the switching means, a multiplexer for switching and thereby outputting the first electric signal retained by the first signal retaining means and the second electric signal retained by the second signal retaining means respectively, and converting means for converting the first electric signal and the second electric signal outputted from the multiplexer respectively into digital signals and thereby outputting the digital signals.
Here, the second signal detecting device performs the so-called digital correlated double sampling. Accordingly, it is preferred that the second signal detecting device conducts signal detection in accordance with the second signal detecting method.
Moreover, in the second signal detecting device, the integrating amplifier, the first signal retaining means and the second signal retaining means may be respectively provided on each of line electrodes of an image detector. Here, the image detector is formed by serially stacking a first electrode layer having transmissivity with respect to an electromagnetic wave for recording, a recording photoconductive layer which takes on conductivity by receiving irradiation of the electromagnetic wave for recording transmitted through the first electrode layer, an electric accumulator for accumulating electric charges generated in the recording photoconductive layer by irradiation of the electromagnetic wave for recording as electric charges for a latent image, a retrieving photoconductive layer which takes on conductivity by receiving irradiation of an electromagnetic wave for retrieval, and a second electrode layer having a stripe electrode composed of a plurality of line electrodes being arranged in given pitches for retrieving the electric charges for a latent image from the electric accumulator. Here, in the second signal detecting device, the multiplexer may switch the first electric signal and the second electric signal concerning the charge signal outputted from the respective line electrodes depending on the respective line electrodes and thereby may output the first and second electric signals to the converting means.
A third signal detecting device of the present invention includes an integrating amplifier for accumulating charge signals and for outputting electric signals corresponding to amounts of the accumulated electric charges, switching means for switching the integrating amplifier between an accumulator mode and a reset mode, first signal retaining means for retaining a first electric signal outputted immediately after the integrating amplifier is switched to the accumulator mode, second signal retaining means for retaining a second electric signal outputted from the integrating amplifier immediately before the integrating amplifier is switched to the reset mode by the switching means, first converting means for converting the first electric signal retained by the first signal retaining means into a digital signal, and second converting means for converting the second electric signal retained by the second signal retaining means into a digital signal.
Here, the third signal detecting device performs the so-called digital correlated double sampling as similar to the second signal detecting device. Accordingly, it is preferred that the third signal detecting device conducts signal detection in accordance with the third signal detecting method.
Moreover, in the third signal detecting device, the integrating amplifier, the first signal retaining means and the second signal retaining means may be respectively provided on each of line electrodes of an image detector. Here, the image detector is formed by serially stacking a first electrode layer having transmissivity with respect to an electromagnetic wave for recording, a recording photoconductive layer which takes on conductivity by receiving irradiation of the electromagnetic wave for recording transmitted through the first electrode layer, an electric accumulator for accumulating electric charges generated in the recording photoconductive layer by irradiation of the electromagnetic wave for recording as electric charges for a latent image, a retrieving photoconductive layer which takes on conductivity by receiving irradiation of an electromagnetic wave for retrieval, and a second electrode layer having a stripe electrode composed of a plurality of line electrodes being arranged in given pitches for retrieving the electric charges for a latent image from the electric accumulator. Here, the third signal detecting device may also include a first multiplexer for switching the first electric signal concerning the charge signal outputted from the respective line electrodes depending on the respective line electrodes and thereby outputting the first electric signal to the first converting means, and a second multiplexer for switching the second electric signal concerning the charge signal outputted from the respective line electrodes depending on the respective line electrodes and thereby outputting the second electric signal to the second converting means.
According to the first signal detecting method and the first signal detecting device of the present invention, a signal detecting method and a signal detecting device are provided for performing the so-called analog correlated double sampling, in which the signal component being found concerning the first charge signal is retained and the retained signal component is converted into a digital signal. Then, the integrating amplifier is switched to the accumulator mode after completing accumulation concerning the first charge signal but before completing conversion of the signal component into the digital signal so as to initiate accumulation concerning the second charge signal. Accordingly, it is possible to initiate accumulation concerning the second charge signal without waiting for completion of conversion of the signal component into the digital signal. In this way, it is possible to shorten the process time correspondingly. Otherwise, if the shortened time is allotted to accumulation by the integrating amplifier, it is possible to lengthen the time for retrieving the charge signal. Accordingly, it is possible to enhance an S/N ratio of the signal to be detected.
Moreover, in the case of retrieving a radiation image by scanning a solid state radiation image detector with retrieving light, for example, it is also possible to perform accumulation of the charge signal during the conversion without overlooking the charge signal retrieved during the conversion of the signal component into the digital signal as observed in the prior art. Accordingly, it is possible to enhance an S/N ratio of the retrieved radiation image.
According to the second signal detecting method and the second signal detecting device of the present invention, a signal detecting method and a signal detecting device are provided for performing the so-called digital correlated double sampling, in which the first electric signal concerning the first charge signal is retained by the first signal retaining means, and the second electric signal concerning the first charge signal is retained by the second signal retaining means. Moreover, the integrating amplifier is switched to the accumulator mode after completing accumulation concerning the first charge signal but before completing conversion of the retained second electric signal into the second digital signal so as to initiate accumulation concerning the second charge signal. Accordingly, it is possible to initiate accumulation concerning the second charge signal without waiting for completion of conversion of the second electric signal into the digital signal. In this way, it is possible to shorten the process time correspondingly. Otherwise, if the shortened time is allotted to accumulation by the integrating amplifier, it is possible to lengthen time for retrieving the charge signal. Accordingly, it is possible to enhance an S/N ratio of the signal to be detected.
Moreover, in the case of retrieving a radiation image by scanning a solid state radiation image detector with retrieving light, for example, it is also possible to perform accumulation of the charge signal during the conversion without overlooking the charge signal retrieved during the conversion of the second electric signal into the digital signal as observed in the prior art. Accordingly, it is possible to enhance an S/N ratio of the retrieved radiation image.
According to the third signal detecting method and the third signal detecting device, a signal detecting method and a signal detecting device are provided for performing the so-called digital correlated double sampling, in which the first electric signal is retained by the first signal retaining means, and the retained first electric signal is converted into the first digital signal by the first converting means. Then, the second electric signal is retained by the second signal retaining means, and the retained second electric signal is converted into the second digital signal by the second converting means. Moreover, the integrating amplifier is switched to the accumulator mode after completing accumulation concerning the first charge signal but before completing conversion of the retained second electric signal into the second digital signal so as to initiate accumulation concerning the second charge signal, and the first electric signal concerning the second charge signal outputted from the integrating amplifier immediately after the integrating amplifier is switched to the accumulator mode. Thereafter, the first electric signal concerning the second charge signal is converted into the first digital signal and outputted before completion of conversion of the second electric signal concerning the first charge signal into the second digital signal. Accordingly, as similar to the second signal detecting method and the second signal detecting device, it is possible to initiate accumulation concerning the second charge signal without waiting for completion of conversion of the second electric signal into the digital signal. In this way, it is possible to shorten the process time correspondingly.
In addition, conversion of the second electric signal concerning the first charge signal into the digital signal and conversion of the first electric signal concerning the second charge signal into the digital signal are respectively performed in parallel by the different converting means. Accordingly, it is possible to lengthen time for conversion processing correspondingly. In this way, it is possible to use relatively low-speed and low-cost devices as the converting means.